1. Field of the Invention
This invention relates to fuel cells having a nickel-based anode and cathode and a molten carbonate electrolyte within a porous ceramic matrix and, in particular, to an anode for use in a fuel cell having a structure that enhances electrolyte retention. More particularly, the invention comprises an anode for use in a molten carbonate fuel cell that is formed with ceramic additives using a method that is easy and inexpensive to perform and repeat. The resulting anode structure has a high capacity for electrolyte, which limits electrolyte loss, which in turn improves fuel cell stability and enhances the lifetime of the cells, and which also reduces cathode flooding as well as cathode polarization.
2. Description of the Related Art
A fuel cell is a device that directly converts chemical energy in the form of a fuel into electrical energy by way of an electrochemical reaction. In general, like a battery, a fuel cell includes a negative electrode or anode and a positive electrode or cathode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. In order to produce a useful amount of power, individual fuel cells are typically arranged in stacked relationship in series with an electrically conductive separator plate between each cell.
Fuel cells having molten carbonate electrolyte are of particular interest for power generation due to their high efficiency and clean conversion of chemical energy into electrical energy. Carbonate fuel cells operate at intermediate temperatures, or approximately 575-700° C., and use carbonaceous fuel containing carbon dioxide and carbon monoxide. A common fuel cell assembly includes a porous nickel (Ni) anode which has been stabilized against sintering by chromium or aluminum additives, or both, and a porous in-situ oxidized and lithiated nickel oxide (NiO) cathode, separated by molten alkali carbonate electrolyte (either Li2CO3/K2CO3 or Li2CO3/Na2CO3) contained within a porous ceramic matrix (such as LiAlO2).
A common problem and significant consideration in the long-term operation of a carbonate fuel cell is the loss of electrolyte due to electrolyte creep, evaporation and corrosion of fuel cell components. Electrolyte loss causes high internal resistance and ultimately shortens the lifetime of the fuel cell. Various attempts have been made to mitigate this problem.
One method consists of incorporating excess electrolyte in the fuel cell. Such an excess, however, will cause cathode flooding because of the low contact angle of the NiO cathode, which has good wettability. Cathode flooding in turn leads to significantly increased cathode polarization and consequently reduces overall cell performance. To prevent flooding of the cathode, the anode could be formed with smaller pores which by capillary pressure would increase the amount of electrolyte in the anode. On the other hand, smaller pores in the anode may lead to diffusion resistance of the reactant gas, which polarizes the electrodes and decreases the cell performance.
It may therefore be preferable, instead of forming an anode with smaller pores, to improve the wettability of the anode. In response to the need for a molten carbonate fuel cell anode with high porosity and improved wettability, a ceramic film coating has been used on the surface of the anode to improve its wettability. For example, Hong et al. (U.S. Pat. No. 6,824,913) disclose an anode structure developed to improve wettability of the anode to the molten carbonate electrolyte without changing the anode or electrolyte material itself, and is directed to the problem of electrolyte loss in the anode of a molten carbonate fuel cell. In particular, Hong et al. teach an anode coated with a porous ceramic film, which avoids recognized problems with small pores in the anode. The anode is coated with a porous thin film of ceramic materials such as CeO2 and Al2O3 to improve the anode wettability and reduce electrolyte loss.
However, as can be seen in FIG. 2 of Hong et al., preparing the coated anode is a complex technique requiring many steps such as sintering green tape at 1000° C., repeatedly coating and drying, and sintering again until the desired structure is formed. The different processing steps in the technique of Hong et al. are expensive and enhance the risk of brittleness, mainly in the production of large anodes. In addition, the non-conductive coating used in Hong et al. may significantly increase contact resistance.
Therefore, there is a need for an anode having improved wettability and performance characteristics which can be prepared using a simple and a cost-effective technique. In addition, there is a need for a method of forming such an improved anode electrode which reduces the risk is of brittleness of the electrode and which does not negatively affect the electrodes contact surfaces.
It is therefore an object of the invention to provide an improved anode with improved wettability and electrolyte retention characteristics.
It is a further object of the invention to provide a method of preparing the improved anode that is not complex and does not require multiple different processing steps.
It is also an object of the invention to provide a method of preparing the improved anode that is cost-effective and that does not negatively affect the strength and porosity of the anode.